1. Field of the Invention
The present invention relates to sealing compositions and, in particular, to compositions for sealing alumina ceramic packages in which integrated circuits or other solid state devices are encapsulated.
A known sealing composition for sealing alumina ceramic packages comprises a mixture of vitreous solder glass powder, lead titanate powder, and zircon (zirconium silicate, ZrSiO.sub.4) powder, as disclosed in Japanese Patent Publication No. 56-49861. This known sealing composition has good mechanical strength and resistance to thermal shock. However, it has a high dielectric constant and, therefore, is not good for sealing packages containing large-scaled integrated circuits such as memories. Further, the composition disadvantageously radiates .alpha.-rays because zircon usually available includes radioactive impurities such as uranium and/or thorium therein whereby certain integrated circuits are damaged.
U.S. Pat. Nos. 4,405,722 and 4,421,947 disclose sealing compositions including zircon which is not desirable for the above-described reason.
U.S. Pat. No. 4,405,722 discloses a mixture of vitreous glass powder and cordierite powder including .beta.-eucryptite, .beta.-spodumen, zircon, and/or lead titanate as optional ingredients. The sealing composition, even if zircon is not used, is not high in mechanical strength, and has a tendency to suffer from micro-cracks caused by heat shock resulting in damage to the hermetical seal.
U.S. Pat. No. 4,421,947 discloses a sealing composition having crystalline filler other than zircon, but the sealing composition is not a powdery mixture and has a high sealing temperature of about 1225.degree. F.
Another known sealing composition comprises a mixture of a PbO-B.sub.2 O.sub.3 or PbO-B.sub.2 O.sub.3 -ZnO solder glass powder and non-inert zinc zirconium silicate powder, as disclosed in U.S. Pat. No. 3,963,505. However, the sealing composition is not so desirable because zinc zirconium silicate included therein includes radioactive impurities such as uranium and/or thorium.